SIMULATION OF THE DECOMPOSITION AND NITROGEN MINERALIZATION OF ABOVEGROUND PLANT-MATERIAL IN 2 UNFERTILIZED GRASSLAND ECOSYSTEMS

A simple model of the decomposition and nitrogen mineralization of pla nt material from two unfertilized grassland ecosystems has been develo ped, with only the proportion of leaves and stems in the original mate rial, the initial nitrogen contents of these plant parts and temperatu re as input data. The model simulates carbon losses from stems and lea ves, using a double exponential decay function, with the temperature s um as independent variable. Mineralization of nitrogen is not calculat ed via microbial growth rates, but simulated on the basis of the carbo n utilization efficiency of the microorganisms and the critical C/N ra tio, i.e. the C/N ratio of the litter at which the microbial demand fo r nitrogen is met exactly. The parameter values for leaching fractions of carbon and nitrogen, relative decay rates, microbial carbon utiliz ation efficiencies and critical C/N ratios were derived from a litter bag experiment with 12 litter types (species) including both green and dead materials, carried out in two unfertilized grassland ecosystems differing in production level. The model was evaluated using a cross-v alidation method, in which one species was omitted from the parametriz ation procedure, and its decomposition and mineralization were predict ed by the resulting model. In general there was good agreement between the observed and predicted amounts of carbon and nitrogen remaining f or ail green litter types/species, but carbon and nitrogen dynamics in the dead material of Festuca rubra were poorly predicted. This dispar ity has been attributed to the proportion of leaves in the material of Festuca rubra (95%) being far beyond the range of leaf proportions in the three litter types the calibration set consisted of (8-35%). When the data of all litter types were used to determine the model paramet ers, good agreement was obtained between measured and simulated values for the changes in nitrogen and carbon in all litter types of both th e green and dead material series. Optimization yielded parameter value s for microbial carbon utilization efficiencies of 0.30 for microorgan isms associated with green litter and 0.35 for those associated with d ead litter. The critical C/N ratios for green and dead material were f ound to be 29 and 36, respectively.